1. Field of the Invention
The present invention relates to an AC-DC converter which is improved to ensure safe start up and to increase the power factor, while avoiding increase in the number of parts or component of the circuit.
2. Description of the Related Art
Rectifier/smoothing circuit of capacitor-input type has been used as means for rectifying A.C. input of AC-DC converter circuit of various types of electronic apparatuses, by virtue of its simple construction. In recent years, however, a problem has been reported in which higher harmonics are undesirably generated in commercial power line by electronic apparatuses having rectifying/smoothing circuits of capacitor-input type, resulting in erroneous operation of other electronic apparatuses connected to the same power line.
This has given a rise to the demand for power-factor improved AC-DC converter in which generation of higher harmonics is suppressed. To cope with this demand, AC-DC converters have been proposed which employs various measures for improving power factor.
FIG. 3 shows the circuit of a known AC-DC converter of improved power factor type. This AC-DC converter has the following circuit construction. A rectifier circuit 8 is connected to a commercial power line through input terminals 11A, 11B. The output of the rectifier circuit 8 is connected to the input terminal of a booster converter circuit 9 the output of which is connected to the input of a DC-DC converter circuit 10. The output of the DC-DC converter 10 is connected to an external load through output terminals 12A, 12B.
The above-mentioned booster converter circuit 9 is of the type known as "active filter".
More specifically, the rectifier circuit 8 has a rectifier 1 having A.C. input terminals connected to the above-mentioned input terminals 11A, 11B. The positive rectified output terminal of the rectifier 1 is connected to the primary winding L1 of the transformer 5 of the booster converter circuit 9 through a rush current prevention resistor R1, while the negative rectified output terminal of the same is connected to the low-potential line which is formed by the booster converter circuit 9, DC-DC converter circuit 10, and output terminal 12B.
The booster converter circuit 9 has the following circuit construction.
One end of the primary winding L1 of the transformer 5 is connected to the rush current prevention resistor R1 of the rectifier circuit 8, while the other end of the primary winding L1 is connected to the anode of a diode D1. The primary winding L1 functions also as a choke coil during operation of the booster converter circuit 9.
The cathode of the diode D1; serving as the output end of he booster converter Circuit 9, is connected to the DC-DC converter circuit 10.
An output capacitor C1 is connected between the cathode of the diode D1 and the low-potential line. A switching transistor Q1 has a principal current path which is connected between the anode of the diode D1 and the low-potential line.
A booster converter control circuit 2 has a control output terminal which is connected to the gate of the transistor Q1, while a voltage detecting terminal of the same is connected to the positive terminal of the output capacitor C1. The booster converter control circuit 2 also has a grounding terminal which is connected to the low-potential line.
The secondary winding L2 of the transformer 5 has one end connected to the low-potential line, while the other end is connected to the power input terminal of the booster converter control circuit 2 via the diode D2. The forward direction of the diode D2 is determined such that current is allowed to flow from the secondary winding L2 to the booster converter control circuit 2.
A series connection composed of a resistor R2 and a diode D3 is connected between the terminal of the primary winding L1 of the transformer 5 connected to the rectifier circuit 8 and the power input terminal of the booster converter control circuit 2. The forward direction of the diode D3 is so determined that the current flows from the resistor R2 to the booster converter control circuit 2.
The series connection of the resistor R2 and the diode D3, connected to the power input terminal of the booster converter control circuit 2, serves as a power supply circuit when the booster converter control circuit 2 is started. The series connection formed by the secondary winding L2 and the diode D2 serves as a power supply circuit which continues the supply of the power to the booster converter control circuit 2 after the start up of the same, Thereby keeping the booster converter control circuit 2 operative.
The secondary winding L2 of the transformer 5, the resister R2 and the diodes D2, D3 in cooperation form a booster converter driving power supply 6A for driving the booster converter.
The DC-DC converter circuit 10 has the following circuit construction. The DC-DC converter circuit 10 has a converter transformer 4 having a primary winding N1, a secondary winding N2 and a ternary winding N3. One end of the primary winding N1 is connected to the output terminal of the booster converter circuit 9, while the other end of the primary winding N1 is connected to the low-potential line through a principal current path of the switching transistor Q2.
A rectifying/smoothing circuit composed of a rectifier diode D6, a fly-wheel diode D7, a choke coil L3 and a smoothing capacitor C2 is connected to the secondary winding N2 of the converter transformer 4. Both ends of the smoothing capacitor C2 are connected to the output terminals 12A and 12B.
The DC-DC converter control circuit 3 has a control output terminal which is connected to the gate of the switching transistor Q2, while the voltage detecting terminal of the same is connected to the positive terminal of the smoothing capacitor C2. The grounding terminal is connected to the low-potential line.
The ternary winding N3 of the converter transformer 4 is connected at its one end to the low-potential line, while the other end of the same is connected to the power supply input terminal of the DC-DC converter control circuit 3 through a diode D5. The diode D5 has such a forward direction that electrical current flows from the ternary winding N3 to the DC-DC converter control circuit 3. A series connection including a resistor R3 and a diode D4 is connected between the terminal of the primary winding N1 of the converter transformer 4 connected to the booster converter circuit 9 and the power input terminal of the DC-DC converter control circuit 3. The diode D4 has such a forward direction that the current flows from the resistor R3 to the DC-DC converter control circuit 3.
The series connection of the resistor R3 and the diode D4, connected to the power input terminal of the DC-DC converter control circuit 3, forms a power supply circuit which supplies power to the DC-DC converter control circuit 3 when the latter is started, while the series connection composed of the ternary winding N3 and the diode D5 functions as a power supply circuit which supplies power to the DC-DC converter control circuit 3 to keep it operative after the start.
The ternary winding N3 of the converter transformer 9, resistor R3, and the diodes D4, D5 in cooperation form a DC-DC converter driving power supply circuit 7 for driving the DC-DC converter.
A brief description will now be given of the operation under normal condition of the AC-DC converter shown in FIG. 3. The AC input from the commercial power line is rectified by the rectifier circuit 8 the output of which is supplied to the booster converter circuit 9. The booster converter circuit 9 then generates, as a result of functioning of the primary winding L1 of the transformer 5 and the switching transistor Q1, a voltage greater than the rectified output voltage of the rectifier circuit 8. This voltage is applied across the output capacitor C1 to charge it. The DC-DC converter circuit 10 receives energy from the output capacitor C1 and converts it into a D.C. stabilized output which is then outputted through output terminals 12A, 12B.
The known AC-DC converter, through the operation described above, provides a stable output and, in addition, does not produce higher harmonics in the commercial power line, thanks to the high power factor of the circuit.
The reason why the circuit construction shown in FIG. 3 improves the power factor is not described because such reason is well known to those skilled in the art in regard to improvement in power factor of booster converter circuits which are generally referred to as, for example, "active filter".
The following problem is encountered when the AC-DC converter circuit is started. Firstly, in general, the rectifier circuit 8, booster converter circuit 9 and the DC-DC converter circuit 10 are started in the mentioned order in accordance with the direction of flow of the electric energy. Namely, the rectifier circuit 8 is started first, followed by the operation of the booster converter circuit 9. In order that the booster converter circuit 9 can start by itself from the beginning, it is necessary to employ a booster converter driving power supply circuit 6A which has a function to start the booster converter control circuit 2.
The second problem is as follows. The output capacitor. C1 is charged after the operation of the booster converter circuit 9. It is necessary that the starting of the DC-DC converter circuit 10 is conducted after the voltage across the output capacitor C1 has been raised to exceed a predetermined voltage level. It is therefore necessary that the DC-DC converter driving power supply 7 for driving the DC-DC converter control circuit 3 or, alternatively, the DC-DC converter control circuit 3 itself, is provided with such a function that allows the DC-DC converter circuit 10 to start only after the voltage charged in the output capacitor C1 has sufficiently exceeded the minimum operation input voltage of the DC-DC converter circuit.
The following problem is caused when such a function is lacked. Assume here that the DC-DC converter circuit 10 has started when the voltage charged in the output capacitor C1 is substantially equal to the minimum operation input voltage of the DC-DC converter circuit 10. In such a case, the rise of the voltage charged in the output capacitor C1 stagnates due to balance between the impedance of the booster converter circuit 9 and the energy supplied to the DC-DC converter circuit 10, with the result that the output of the DC-DC converter circuit 10, which has to be stabilized, cannot be stabilized satisfactorily.
Due to the two major problems described above, the AC-DC converter of the type incorporating the power-factor improved circuit including the booster converter circuit essentially has a more complicated construction than ordinary AC-DC converter which employs a capacitor-input type rectifying/smoothing circuit, with the result that the dimensions and the cost are increased in the AC-DC converter of the type mentioned above.
In particular, the second problem impairs the reliability of operation of the AC-DC converter at the time of stare up of the converter.